Product and method for making same

ABSTRACT

One-piece, light-weight, transparent telescope mirror blanks consisting essentially of one integral shaped mass of transparent, low-expansion, at least partially crystallized glass ceramic having a pair of oppositely disposed face surfaces and a plurality of separate cavities between the face surfaces. One of the face surfaces of the mirror blank has a plurality of openings therein, each of the openings being in communication with a cavity. The cross-sectional area of the opening is smaller than the cross-sectional area of the cavity. Telescope mirrors can be made by coating the surface of the mirror blank with an appropriate reflecting coating. Methods of making the mirror blank and mirror are also disclosed as well as apparatus.

United States Patent Austin et al.

PRODUCT AND METHOD FOR MAKING SAME Inventors: Lewis I M. Austin; RobertR. Denman; Thomas P. ODonnell; Frank Veres, all of Toledo, OhioAssignee: Owens-Illinois, Inc.

Filed: July 23, 1970 Appl. No.1 63,989

Related U.S. Application Data Division of Ser. No. 862,122, Aug. 5,1969, which is a division of Ser. No. 468,691 July I, 1965, Pat. No.3,484,328, which is a continuation-in-part of Ser. No. 437,431, March 5,1965, abandoned.

U.S. CI ..350/310, 350/288 Int. Cl. ..G02b 7/18 Field of Search..350/288, 310

References Cited FOREIGN PATENTS OR APPLICATIONS 8/1964 Great Britain..350/310 1 Jan. 30, 1973 OTHER PUBLICATIONS Mann et al., A NewUltra-Expansion, pp. 819-822, Modified Fused Silica Glass, May1968,'Applied Optics, Vol.7, No. 5.

Primary ExaminerDavid Schonberg Assistant Examiner-Michael J. TokarAttorney-W. A. Schaich et al.

[57] ABSTRACT is smaller than the cross sectional area of the cavity.-

Telescope mirrors can be made by coating the surface of the mirror blankwith an appropriate reflecting coating. Methods of making the mirrorblank and mirror are also disclosed as well as apparatus.

2 Claims, 11 Drawing Figures PATENTEDJAH 30 ma SHEET 10F 5 EM" Z1PATENTED JAN 30 I975 SHEET Q 0F 5 W W a w PRODUCT AND METHOD FOR MAKINGSAME This application is a division of copending application Ser. No.862,122, filed Aug. 5, 1969, which in turn is a division of copendingapplication Ser. No. 468,691, filed July 1, 1965, now U.S. Pat. No.3,484,328, which in turn is a continuation-in-part of application Ser.No. 437,431, filed Mar. 5, 1965, now abandoned.

This invention relates to a telescope mirror blank and telescope mirrormade therefrom, and, more particularly, to a one-piece light-weight,transparent, lowexpansion telescope mirror blank, to a process formaking the same, and to the light-weight telescope mirror madetherefrom.

Telescope mirror blanks of large diameters and thicknesses presentspecial and difficult problems in the casting thereof and, after thereflecting surface has been carefully ground, polished, figured, andcoated, in the subsequent handling and mounting of these mirrors in thetelescopes. Due to the enormous weight of the larger mirrors, complexmounting means must be utilized for supporting the mirror in thetelescope, care being taken to assure that the mirror is always instrainfree condition, irrespective of its position as the telescope ismoved to observe different areas of the sky.

Since the condition and configuration of the reflecting surface of thefinished mirror determines the accuracy of the reflected image, andsince the mirror must be of sufficient rigidity so as to prevent eventhe slightest movement or distortion of the reflecting mirror surface,the thickness of the mirror must be substantial in order to insure suchrigidity. Attempts have been made in the past to lighten the overallweight of such telescope mirrors by forming a mirror blank of a minimumthickness and then subsequently bonding the under surface of the mirrorblank to glass members of the same composition, which members, as awhole, impart a certain rigidity to the ultimate reflecting mirror.

One example of this is the use of an egg-crate" construction wherein aplurality of elongated glass strips having spaced slotted portions alongone longitudinally extending edge are interconnected with a plurality ofsimilar glass strips extending at right angles thereto, the connectionbeing at the respective slotted portions so that the final structure isof the same thickness or height as the individual strip, just as in thecase of the interconnected cardboard members used with an egg crate toseparate the individual eggs.

However, due to the overall size of the glass strips and the thicknessof the mirror blank, considerable problems occur when the glass stripsare subjected to temperatures sufficient to fuse them together alongadjoining portions and also fuse the upper surface of the interlockedegg-crate structure to the bottom surface of the mirror blank. A glassback-up plate of sufficient thickness to impart rigidity to the mirrormust also be fused to the other surface of the egg-crate" structure.Since the casting of large mirrors is a very delicate operation, and thetemperature to which the glass is subjected as it is being cooled has tobe very carefully controlled, it will be apparent that serious problemsoccur as soon as an attempt is made to fuse to the egg-crate glassstructure the bottom surface of the glass mirror blank and to theback-up plate. To do this successfully is a tedious, time-consumingoperation which adds considerably to the cost of the final product.

Accordingly, it is an object of this invention to provide a process forforming a one-piece light-weight, reflecting, vitreous mirror for atelescope, which process avoids the disadvantages which exist in theprior art.

It is another object of this invention to provide a onepiecelight-weight, low-expansion, vitreous telescope mirror blank wherein thesurface of the blank can be ground, polished, figured and coated,whereby the resulting mirror, when mounted within a telescope, is of arigidity sufficient to prevent any distortion of the reflecting surface.

Another object of this invention is to provide a lightweight,transparent, low-expansion, thermally crystallized glass-ceramictelescope mirror blank having a low lineal coefficient of thermalexpansion.

Still another object of this invention is to provide a one-piecelight-weight low expansion vitreous telescope mirror blank having aplurality of separate cavities disposed through out its interior, eachcavity communicating to the atmosphere through small openings in themirror blank surface, the volume of these cavities being such that theoverall weight of the mirror blank, and mirror which is made therefrom,is considerably reduced.

Since another object of this invention is to provide a process formaking a one-piece, light-weight, transparent thermally crystallizedglass-ceramic telescope mirror blank having a low coefficient of linealthermal expansion wherein the mirror blank has a plurality of separatecavities disposed throughout its interior and communicating to theatmosphere through small openings in one surface of said blank, thevolume of these cavities being such that the overall weight of saidmirror blank is considerably reduced.

Still another object of this invention, is to provide a process formaking a light-weight, low-expansion, telescope mirror blank ofsubstantial thickness and diameter, which may be readily cast and heattreated to the desired degree in a minimum length of time and at arelatively low cost.

In attaining these objects, one feature resides in forming a vitreousmirror blank having a pair of oppositely disposed face surfaces and aplurality of separate cavities disposed between the surfaces. One of thesurfaces is providedwith a plurality of openings, each of which isdisposed above or opposite one of the cavities and communicates with thecavity. Each opening is considerably smaller in cross-sectional areathan the cross-sectional area of the cavity.

Another feature resides in forming a transparent glass ceramic telescopemirror blank by maintaining a molten thermally crystallizable glasswithin a zone, which glass is preferably of the Si0 Al 0 Li 0 system,maintaining a plurality of shaped modules, each of which has a supportmember affixed thereto, in the molten zone, and having each of themodules being completely immersed in the molten glass. By increasing theviscosity of the glass until it is self-supporting, removing theself-supporting glass from the zone and subjecting it to "a particularheat treatment until the glass is thermally crystallized in situ, a lowexpansion transparent telescope mirror blank is produced containing aspredominant crystalline phases lithium-containing crystalline phases,either as beta-eucryptite or beta-eucryptite-like crystals, or asbeta-spodumene or beta-spodumene-like crystals, or both, as indicated byX-ray diffraction data. A multitude of such crystalline phases in randomorientation and dispersed in the glassy matrix remaining as a result ofin situ crystallization are to be found in the glass ceramic mirrorblank of the invention. Substantially all of the crystals of theglassceramic are of a diameter less than one-third micron measured alongthe largest lineal dimension of the crystals. Upon removal of themodules from the interior of the blank, a plurality of cavities areformed therein.

Still another feature of this invention is to form a light-weight,transparent, glass-ceramic telescope mirror blank wherein the blank hasa coefficient of lineal thermal expansion of from minus 10 X 10- to 10 X10 760 a a 013 c)smarter-any6556M zrdJdE pending upon the compositionand the heat treatment to which it is subjected.

Other objects, features and advantages of the invention will become moreapparent from the following discussion of the invention, taken inconjunction with the drawings, wherein:

FIG. 1 is a cross-sectional view of one portion of the telescope mirrorblank of the present invention, shown cast in a mold and having themodules present within the blank;

FIG. 2 is a cross-sectional view of the one-piece telescope mirror blankof the present invention showing the plurality of cavities therein;

FIG. 3 is an enlarged plan view of the underside of the annulartelescope mirror blank of the invention illustrated in FIG. 1 showingthe plurality of openings disposed above the individual cavities, andshowing'the cavities in the broken-away portion of the mirror;

FIG. 4 is a cross-sectional view of another embodiment of the telescopemirror blank of the present invention;

FIG. 5 is a perspective view of one form of apparatus which can beutilized in making the telescope mirror blank of the present invention.

FIG. 6 is a cross-sectional view of a mold of the type illustrated inFIG. 5 shown in a closed position with the individual modules supportedwithin the molten glass;

FIG. & is a fragmentary section of a mold of the type illustrated inFIG. 6 and showing a perspective view of a module supported within themolten glass.

FIG. 8 is a cross-sectional view of the upper portion of the moduletaken along lines 88 of FIG. 7;

FIG. 9 is a perspective view of another embodiment of the telescopemirror blank of the present invention showing the support members stillaffixed to the modules disposed within the interior of the blank.

FIG. 10 is a fragmentary cross-sectional view of still anotherembodiment of the telescope mirror blank of Removably secured to eachpin is a shaped module or cavity-forming unit 13 having a shoulderportion 14 and a body portion 15. As shown in FIG. I, the module has aneck portion 16 integral with the shoulder 14 and removably disposedabout pin 11 so that the neck portion 16 completely shields pin 11 fromthe molten vitreous mass 9.

The shaped module can be of any material which is resistant to the hightemperature of the molten glass and which retains its shape during thesubsequent heat treatment process. The material should be of a typewhich has a low coefficient of expansion and which can be readilyremoved from the interior of the mirror blank after'the blank is formed.One of many suitable materials for this purpose is a shaped, open celltype amorphous fused silica foam formed by slip casting the silica inplaster molds to the desired shape. Suitable shaped modules can beformed by utilizing the aforesaid silica which is sold under thetrademark Glasrock Foam No. by Glasrock Products, Inc. of Atlanta,Georgia. The amorphous silica which is at least 98 percent pure silicawith Al tl being the major impurity, has a linear coefficient of thermalexpansion of 0.54 X 10' per C (0-l,000C) and can withstand thermalshocks of up to 3,100F. The bulk density is between 23 and 28 lb./cu.ft.

After the citreous mirror blank 17 of the invention has been cast,removed from the mold l0, subjected to a prescribed heat treatment andsubsequently cooled to ambient temperature, the shaped modules 13 can bereadily removed from the interior of the blank through the openings 18on the underside of the mirror blank by chipping them with a suitabletool. A mirror blank 17 having a plurality of cavities 19 is thusobtained, as illustrated in FIGS. 2 and 3. FIG. 4 illustrates anotherembodiment of the present invention wherein the cavities 19 can be ofdifferent shapes, depending upon the configuration of the shaped modulesutilized in casting the mirror blank. In the preferred embodiments, thebottom surfaces 20 of each of cavities 19 are equally spaced from thesurface 21 of the mirror blank 17, which surface 21 eventually becomesthe reflecting surface of the telescope mirror. Due to the symmetricalarrangement of the shaped modules in the mold, the resulting mirrorblank as shown in FIG. 3 has a plurality of spaced cavities in parallelrows forming a series of ribs 22 extending in two directions, thethickness of the rib portions at the elevation where openings 18communicate with cavities 19 preferably being less than the distancebetween cavities 19 and surface 21 that will be mirrored.

As illustrated in FIG. 5, a mold 23 having an ocmold 23 is amodule-holding plate 25 having a pair of angle irons 26 secured to itsupper surface 27 and extending outwardly beyond the edges, thuspermitting manual lifting and lowering of the plate 25 relative to mold23. A plurality of angle irons 28 are also secured (by means not shown)to the upper surface 27 of plate 25 and extend at right angles to angleirons 26. Each of angle irons 28 has a plurality of openings 29therethrough (see FIG. 7) and the openings 29 are aligned so as toreceive rods 30 extending therethrough.

As more readily seen in FIG. 7, each of the shaped modules 13 has anannular metal ring 31 on shoulder 14 and surrounding an elongatedsupport member 32 which extends inwardly into module 13. Member 32 andring 31 are secured to the shoulder 14 of module 13 by a suitableadhesive or cement 33, such as sauereisen. Support member 32 extendsupwardly through an opening 34 in plate 25 and the upper end 35 has anopening 36 through which rod 30 extends. The support member 32 are soaffixed into modules 13 that the rings 31 are in contact with the undersurface 37 of plate 25 so that no molten glass is permitted to come intocontact with support member 32.

In utilizing the apparatus of FIG. 5, a molten vitreous mass is firstpoured into mold 23 to a predetermined level. Plate 25 containing theplurality of suspended modules is then lowered until the corners andedges of under surface 37 rest upon the comers 41 and edges 42 of mold23. Modules 13 are thus immersed within the molten vitreous mass and assoon as the viscosity of the mass is increased to a point where the massis self-supporting, the rods 30 are pulled out of openings 36 in supportmember 32, thus permitting plate 25 to be lifted from mold 23. All ofsupport members 32 remain secured to the modules 13 within the vitreousmass, as shown in FIG. 9, where the vitreous mass is shown removed fromthe mold. The mass then undergoes a heat treatment, the temperature andtimes of which are governed by the composition of the mass and theultimate properties which are desired.

When the resulting mirror blank 17 is finally at ambient temperature,the cement 33 holding each of the support members 32 and rings 31 isremoved, members 32 and rings 31 are lifted out and the modules 13chipped away through the opening remaining in the upper surface of theblank until only the cavities remain in the mirror blank. Since themodules 13 supported in plate 25 are of the same height, the flat bottomportions 43 are maintained equally distant from the bottom surface 24 ofmold 23, thus assuring that the thickness of the mirror blank from thebottom 20 of the resulting cavities to the outer surface 21 issubstantially the same throughout the blank (See FIG. 2).

FIG. 6 illustrates another embodiment of the apparatus of FIG. 5 whereina split ring mold 44 having an independent convex bottom surface 45,after it has been filled to a predetermined level with a molten vitreousmass 46, has plate 47 lowered thereon so that shaped modules 48 aresuspended within the vitreous mass. As previously discussed with respectto FIGS. 5 and 7, the modules 48 are supported by members 49 extendingtherein and, in turn, supported by rod 50 passing through openings 51 inmembers 49 and through openings 52 in angle irons 53. In view of theconcave configuration which is imparted to surface 54 of the mass 46,the bottom surfaces 55 of the modules are likewise of the sameconfiguration so that all of bottom surfaces 55 lie in a plane which issubstantially parallel to concave surface 54 and convex mold surface 45.

In still another embodiment of the present invention illustrated in FIG.10, mirror blank 56 is formed with a concave reflecting surface 57 and aconvex back surface 58. This is accomplished by utilizing modules ofvarying shapes suspended from an upper plate similar to plate .47 shownin FIG. 6 except that its bottom surface is of concave configuration andsnugly fits within the mold, defining a zone corresponding to the outerconfiguration of blank 56.

Mirror blank 56 is also formed with a centrally disposed cavity 59 whichcan be of any desired configuration so as to receive mounting means (notshown) therein, thus permitting mounting of the mirror formed from theblank in the manner of a radio or radar antenna. By having the cavities60 of varying sizes and shapes, the mirror blank which is formed has aplurality of radially extending ribs 62 which are of the same thicknessas the glass blank and impart rigidity thereto. By making the mirrorblank of the configuration shown in FIG. 10, the overall weight of themirror formed therefrom is greatly decreased due to the progressivereduction in the thickness of the peripheral portions thereof relativeto its center portion. Again, openings 61 disposed opposite the cavities60 and communicating therewith, permit the cavities to remain atatmospheric pressures and temperatures.

The following example is merely illustrative of an embodiment of theinvention, and it is to be understood that the scope of the invention isnot to be considered limited in any manner thereby.

EXAMPLE I A thermally crystallization molten glass having a temperaturewithin the range of 2,650 2,750F. was poured into a split-ring graphitemold having a convex bottom surface, which mold had been preheated to400F. The mold being 16 inches in diameter, it took approximately 15seconds to pour the molten glass therein to the desired depth. As soonas the glass flow stopped, the upper plate of the mold having a symmet'rical arrangement of a plurality of shaped Glasrock modules of theconfiguration shown in FIG. 8 suspended from its bottom surface waslowered onto the mold so that all of the modules were immersed below themolten glass surface in the manner shown in FIG. 6. The modules had beenpreviously heated to about 800F prior to their immersion into the moltenglass. After 5 seconds had elapsed from the immersion of the modules,the rods supporting the modules were removed from the plate and theplate lifted from the mold. As soon as the viscosity of the glassincreased to a point where the glass was self-supporting, the splitringmold was opened and the glass assembly was supported by the convexbottom surface of the mold. A total time of about 2% to 3 minutes hadelapsed from the time the glass had first been poured into the mold.

The self-supporting glass assembly together with the mold bottom waspromptly placed into an oven which had been preheated to a temperatureof 1,000F and whose temperature was increased to l,I50F due to thepresence of the hot assembly therein, and the assembly was maintained inthe oven for a period of three hours at this temperature.

The temperature in the oven was then increased to 1,350F at the rate ofabout 5F. per minute, and the assembly maintained therein for 50 hoursat this temperature. At the end of this time, the assembly was cooled atthe rate of lF. per minute until the temperature of l,000F was reached,and then the cooling rate was increased to slightly less than F perminute until room temperature was reached. A transparent thermally insitu crystallized glass-ceramic mirror blank 3 inches thick was formed.After the support members had been removed and the Glasrock moduleschipped and scraped away, the mirror blank had a plurality of cavitiesdisposed throughout its inner portion. It had a coefficient of linealthermal expansion of zero 1 l X C (0-300C EXAMPLE ll A mirror blank wasformed by pouring a thermally crystallizable molten glass having atemperature of about 2,700F into a graphite mold which had beenpreheated to a temperature such as to minimize any heat loss by theglass. The mold was 16 inches in diameter and had a plurality ofGlasrock modules supported thereon in the manner shown in FIG. 1. Ittook about -20 seconds to pour the molten glass into the mold about theshaped, open cell type amorphous fused silica foam modules to a depth ofone-half inch above the tops of the preheated modules. The molten glasswas permitted to remain in the mold for about 4 minutes, at which timethe viscosity of the glass had increased to a point where the glass wasself-supporting.

The mold was then inverted and the glass blank was placed on a transitesurface which was promptly placed into an oven preheated to atemperature of 1,000F and whose temperature was increased to 1,l 50F dueto the presence of the hot assembly therein. The assembly was maintainedin the oven for a period of three hours at this temperature. The oventemperature was then increased to 1,350F at the rate of about 5F perminute and the assembly maintained therein for 50 hours at thistemperature. At the end of this time, the assembly was cooled at therate of lF per minute until the temperature of 1,000F was reached, andthen the cooling rate was increased to slightly less than 5F per minuteuntil ambient temperature was reached.

A transparent, thermally in situ crystallized glassceramic mirror blankwas formed. The fused silica modules were chipped and scraped away fromthe blank resulting in a plurality of cavities disposed throughout theinner surface and conforming to the shape of the modules.

The thermally crystallizable glass used in the foregoing examples, whichproduced a thermally, in situ, crystallized transparent glass-ceramictelescope mirror blank was prepared by first melting together thefollowing batch ingredients expressed in pounds and ounces:

ingredients Parts petalite (l) 405 lbs. l4 oz. Zircon sand (2) l5 lbs.2.5 oz. Alumina (3) 39 lbs. 1 oz. Boric acid (4) 30 lbs. 5 oz.High-calcium limestone (5) 24 lbs. 8.5 oz. Zinc oxide 6 lbs. 5 oz.Lithium carbonate 5 lbs. 5 oz. Titanium dioxide (6) 9 lbs. l.5 oz. Niter1 lb. 4 02. Sodium antimonate 1 lb. ll 02.

(l) 4.2% Li,0, 16.1% Al,0,, 77.7% SiO,, 0.4% Na,0, 0.027% Fe,O and otherminor ingredients, including ignition loss.

(2) Analysis of zircon sand is 33.8% 5H),, 65.5% ZrO,, 0.12% 110,, 0.05%Fe,0,, 0.24% ALO and 0.2% cerium oxide and possibly rare earth oxide.

(3) Weight of Alcoa A-l4 alumina which is illustratively 99.5% A1 0,,0.03% Fe,0,, 0.l0% Na,0, 0.08% SiO,, 0.2% ignition loss at l 100C.

(5) Limestone analyzing 55.25% Cat), 0.25% Mg0,- 0.5% Silk, 0.2% N 0,,0.05% Fe,0,, 0.001% Cr,O 0.03% sulfate (S0,), 0.02% P 0, and an ignitionloss of 43.6%.

(6) Weight of Titanox-GM which is a non-pigmentary grade ofsubstantially pure Till, sold by Titanium Pigment Corporation.

This glass has the following theoretical composition and for an actualtank batch had the following analyzed composition, expressed as variousoxides in weight percent:

The differences between theoretical and actual compositions are believedto be due primarily to alumina pick-up from the refractory of thefurnace and to B 0 and Zn0 losses by volatilization.

Based upon the heat treatment, as described above, that is given to thisglass the glass-ceramic obtained should have a thermal coefficient ofexpansion of minus 0.2 X 10 per "C (0-300C).

Another example of a preferred thermally crystallizable glass which bysuitable heat treatment as described below can produce an astronomicaltelescope mirror blank of glass-ceramic with a thermal coefficient ofexpansion of 0 X 10" per "C (0-300C) is described below. The heattreatment for the glass differs from that described above in that thetemperature of the oven in which the molded glass is placed after themaintenance at l,l50F for a period of 3 hours is raised to l,42SF at therate of about 5F per minute, instead of l,350F, and is maintained at1,425F for 48 hours. Otherwise, the process is that described above forExamples l and II.

This glass is prepared by first melting together the following batchingredients expressed in parts by weight:

(1 Petalite composition is as described above following the tabulationof batch ingredients for the other glass described in detail. (2) Zirconsand is also as described earlier.

(3) Weight of Alcoa A-l4 alumina which is illustratively 99.6% M 0 0.04%Na o. 0. l 2% SiO,, 0.2% ignition loss at l l00C.

(4) Limestone is also as described above.

(5) Weight ofTitanox-GM which is also described above.

This glass has the following theoretical composition and for an actualtank batch had the following analyzed composition, expressed as variousoxides and one chemical element in weight percent:

Theoretical,% Analyzed,% Sit), 67.4 67 .5 AM), 20.9 22.l CaO 2.7 2.6

ZnO Li,0 TiO, ZrO Na O not analyzed.

The differences are believed to be due to alumina pick-up andvolatilization loss in the case of Zn0.

While telescope mirror blanks of the present invention may be formedutilizaing known vitreous compositions which have been shown to besuitable for telescope mirrors in the past, such as fused quartz,borosilicate glass, and the like, it is preferred to utilize thermallycrystallizable glasses of the Si -Al O -Li 0 system, capable of beingthermally, in situ crystallized to form transparent glass-ceramicshaving a coefficient of lineal thermal expansion which is low andpreferably is about zero.

Transparent, low-expansion, glass ceramic telescope mirror blanks may beformed by thermal in situ crystallization of the preferred thermallycrystallizable base glass composition of the present invention, whichcomposition consists essentially of the following components, in theindicated percent limits, based on the total glass composition.

Component Weight Percent SiO, 56-68 Al O 18-27 Li O 3.4-4.5

CaO 0-3 ZnO 0-2 TiO 2 0-6 ZrO, 0-3

MgO 0-3 Na O 0-1 P 0 0-3 (SiO Al o at least 82 (SiO M 0 E 0; B0 86-91(Ca0 MgO ZnO Na,0) 2.5-6 (SiO Al,0 P,O,-, Li,0) no more than 93 (TiOZr0,) 2-6 wherein the ratio of (Ca0 MgO Na O B 0 to Li O is less than2.4 and the ratio of SiO to A1 0 is no more than 3.8, and preferably nomore than 3.3. The SiO content may be increased up to 70 weight percentand good results are obtained.

.For uses of glasses and crystalline ceramics of the invention thatrequire holding the formed glass objects for an extended time intemperature ranges where crystallization can take place, given longenough time, it has been found that the amount of TiO plus ZrO should belimited to a maximum of about 3 weight percent and that the TiO shouldbe limited to about 1.5 percent of the glass composition set forthherein. Usually the range of TiO is from i to 1.5 percent in this aspectof the invention. One such use requiring such low nucleant levels is inmaking very large shaped objects, such as very thick telescope mirrorblanks having large diameters, which blanks require a very longannealing time during which the glass must not prematurely crystallize.

The transparent, crystallized glass-ceramic formed, as was formed inExamples I and Il above, contains as predominant crystalline specieslithium-containing crystalline phases'selected from the group consistingof beta-eucryptite or beta-eucryptite-like crystals, or asbeta-spodumene or beta-spodumene-like crystals, or

both as indicated by X-ray diffraction data. The ceramic contains amultitude of such crystalline species which are in random orientationthroughout theceramic and which are dispersed in the glassy matrixremaining as a result of the in situ crystallization. Substantially allof the crystals of the ceramic are of a diameter less than one-thirdmicron measured along the largest lineal dimension of the crystals. Theglass-ceramic has a lineal coefficient of thermal expansion of aboutminus 10 X 10 to 10 X 10' (0-300C) and, preferably, from -3 to 3 X 10'(0300C). The ultimate telescope mirror blank and telescope mirror formedtherefrom is one in which the lineal coefficient of thermal expansion ofthe glass-ceramic is about zero. Furthermore, while the diameter of thecrystals within the ceramic is preferably less than one-third micronmeasured along the largest lineal dimension of the crystals, it ispreferred that the crystals be of a diameter less than one-fourth micronin size, and best results are evident when the diameter is less thanone-tenth micron in size.

Other transparent, low-expansion, crystallized glassceramics formed bythermal in situ crystallization of a thermally crystallizable base glassare disclosed in copending U.S. application Ser. No. 396,011 filed Sept.14, 1964 now abandoned, and in copending U.S. application Ser. No.386,693 filed July 31, 1964 now abandoned, and in thecontinuation-impart applications subsequently filed with respectthereto, all applications being assigned to the assignee of the presentapplication. 'All of the disclosures in the aforesaid applicationsrelating to thermally crystallizable glass compositions and the processof heat treating said compositions to form transparent, low-expansionglass-ceramics of substantial thickness and diameter are incorporatedherein by reference. As fully disclosed in the aforesaid pendingapplications, the final coefficient of thermal expansion of theglass-ceramic is determined by the composition of the thermallycrystallizable glass and by the particular heat treatment to which it issubjected.

A mirror blank having a concave surface can be made in accordance withthe process disclosed in Example II by inverting the mold containing thehotselfsupporting glass onto a convex surface and then permitting theglass to slump thereon and assume the configuration of the supportingsurface. It will be appreciated that by making a surface of the blankconcave, it will facilitate the grinding, polishing and figuring thereofto the desired configuration.

Furthermore, while graphic molds have been used in making the mirrorblanks of the present invention, it is contemplated that the molds maybe made of materials such as open cell type amorphous fused silica from(Glasrock), low-expansion ceramics, and the like which can be preheatedto temperatures approaching that of the molten vitreous mass. It ispreferred that when mirror blanks are to be made from a thermallycrystallizable glass the modules should have rounded corners and edgesas shown in FIG. 7. This will minimize or prevent uncontrolledcrystallization of the glass occurring at sharp edges or corners.Alternatively, the modules can be annular, conical or of any suitableconfiguration and do not necessarily have to be rectangular.

It is to be understood, of course, that when lightweight telescopemirror blanks are to be formed from fused quartz, borosilicate or othervitreous composition, the best treatment step will vary from that ofExamples I and II above since in situ crystallization of the glass isnot necessary. Such heat treatments are those well known in the mirrorblank art.

In the foregoing description of embodiments of the present invention,molten vitreous composition is poured into the mold. As an alternative,the mold may be filled with vitreous cullet which is then heated to atemperature sufficient to melt it within the mold.

In the case of making mirror blanks of fused quartz, in accordance withthe foregoing teaching of the present invention, a material such asGlasrock would not be suitable. In its place one would use a materialsuch as graphite to make shaped modules 13, 48. To

minimize gas bubbles in the blank due to gas evolution, a vacuum can beimposed on the system. As an alternative to vacuum, any gas bubbles canbe kept in the bottom of the molten quartz in the mold by using apressurized system. When making the mirror of quartz, sand is placed inthe mold having the cavity-forming projects and melted.

As illustrated in FIG. 7, each shaped module 13 is supported by member32 having an opening 36. The support member 32 extends up through rigidplate 25 and as described above extends down in module 13. The ring 31is mounted on module 13 and the space between it and member 32 is filledwith an adhesive cement. Instead of these several parts, namely, module13, support 32, ring 31 and cement 33, it is obvious that the entireshape defined by these assembled components can be provided in the formof a single element, the bottom portion of which provides the shapedmodule 13, the intermediate portion provides the equivalent of ring 31(which is really part of the shaped module), and the upper portion ofsupport member 32. This could be made of Glasrock Foam and of course thetop part would have an opening through it as in the case of opening 36in support member 32. The same is true of modules 48 and members 49which can also be cast in one piece. Accordingly, the cavity-formingmembers of the apparatus of the present invention, when made of onepiece constitute module 13, ring 31, and cement 33. This provides theconfiguration that is shown in FIG. 6 for module 48 which constitutessuch a cavity-forming member. Support 32 and support member 49 extenddown into the cavityforming member with the top portion of the supportbeing utilized to position and maintain the cavity-forming members in amanner shown in FIG. 7, for example.

' A transparent, low-expansion telescope mirror blank of the presentinvention, formed by the process disclosed herein, has its base platesurface, which is preferably concave, ground polished and figured, i.e.,a proper parabolic curve is formed on the surface. A thin coating ofaluminum is then applied to the prepared surface in a conventionalmanner to form the reflecting surface.

In the foregoing description of the apparatus of FIG. it is mentionedthat plate 25 supporting modules 13 is lowered to press modules 13 intothe molten vitreous mass. As an alternative, the apparatus can bemodified by having a relatively large opening in plate 25 so that plate25 supporting modules 13 can be positioned with respect to mold 23 sothat modules 13 are at the proper elevation to form cavities in moltenglass when the latter is poured or cast through the relatively largeopening. This alternative eliminates a glass pressing operation.Instead, the molten glass when poured into the mold flows around modules13 and rings 31 that are already supported in position by plate 25, whenmold 23 is supplied with molten glass in the correct amount by flowingthe molten glass through the opening in plate 25.

In a further alternative embodiment, plate 25 can be replaced by a gridsupport for modules 13 and rings 31. The grid support can be acombination of interlocked rods at a common plane to provide manyopenings in vertical planes between modules 13. Thus the molten glasscan be poured into the mold at many different points to surround modules13 with molten glass until the height of the top surface of the moltenglass is at or near the top of rings 31. Of course, the bottom ofmodules 13 are spaced from bottom 24 of mold 23 to provide a suitablethickness between the face of the mirror blank formed by bottom 24 andthe base of the cavities defined by the bottom of modules 13.

In the foregoing description of the method of the present invention whenusing a thermally in situ crystallizable glass, the molded glass iscooled in the mold until its viscosity increases sufficiently to be selfsupporting at its periphery, then it is removed from the mold, cooledfurther as described and subjected to thermal treatment for the in situcrystallization. Obviously within the scope of the invention, all orpart of the mold may be removed at any later time and, likewise, onlypart of the mold may be removed instead of all of it when the glass uponcooling has increased to the extent that the glass is self supporting atthe periphery.

In this specification, as in the above-mentioned pending applications,the terms beta-eucryptite crystals and beta-eucryptite-like crystalshave been used in an alternative sense. Thus, while beta-eucryptite isoften thought of as the species crystal having one mole of lithia, 1mole of alumina and 2 moles of silica, both terms are used in thisapplication to designate crystalline species having the beta-eucryptitestructure, as shown by X-ray diffraction, but the peaks can be shiftedslightly depending upon whether there is a definite amount of silicapresent other than exactly 2 moles, either more or less silica than the2 moles. Similarly, the terms beta-spodumene crystals andbetaspodumene-like crystals are used alternatively and in a genericsense, specifying crystalline species that have the crystallinestructure of beta-spodumene that contains 4 moles of silica to 1 ofalumina and l of lithia, but with the peaks shifted somewhat when thecrystalline structure contains more or less than 4 moles of silica. Inthe claims, therefore, the terms beta-eucryptite and beta-spodumene areeach used in this generic sense.

While the invention has been discussed in terms of telescope mirrorblanks it will be apparent that other large shaped glass objects orarticles can be formed by the processes of the invention, particularlywhere it is important that such objects or articles be of light weight.Such articles, for example, can be used as structural units, includingbuilding blocks, panels, etc., in the building and constructionindustry.

Various modifications of the present invention will be apparent to thoseskilled in the art. The foregoing description and examples are merelyillustrative and the invention will be limited only by the claims thatfollow.

We claim:

1. A light-weight vitreous telescope mirror formed from a one-piece,light-weight, transparent, vitreous telescope mirror blank consisting ofa single vitreous mass and having a pair of oppositely disposed facesurfaces and a plurality of separate cavities disposed between saidsurfaces, one of said face surfaces being provided with a plurality ofopenings, each of said openings being disposed above and communicatingwith one of said cavities, each of said openings being smaller incross-sectional area than the cross-sectional area of said cavity, thevolume between adjacent cavities being occupied by said vitreous mass,said surface of said blank opposite the surface with said openings andcoated with a light-reflective coating thereon.

2. A light-weight, transparent, glass-ceramic telescope mirror formedfrom a one-piece, lightweight, transparent, at least partiallycrystallized glassceramic mirror blank, consisting of a single mass ofglass ceramic, said mirror blank having a pair of oppositely disposedface surfaces and a plurality of separate cavities disposed between saidsurfaces, one of said face surfaces being provided with a plurality ofopenings, each of said openings being disposed above and communicatingwith one of said cavities, each of said openings being smaller incross-sectional area than the cross-sectional area of said cavity, thevolume between adjacent cavities being occupied by said glass ceramic,said surface of said blank opposite the surface with said openings andcoated with a light-reflective coating thereon.

1. A light-weight vitreous telescope mirror formed from a onepiece,light-weight, transparent, vitreous telescope mirror blank consisting ofa single vitreous mass and having a pair of oppositely disposed facesurfaces and a plurality of separate cavities disposed between saidsurfaces, one of said face surfaces being provided with a plurality ofopenings, each of said openings being disposed above and communicatingwith one of said cavities, each of said openings being smaller incrosssectional area than the cross-sectional area of said cavity, thevolume between adjacent cavities being occupied by said vitreous mass,said surface of said blank opposite the surface with said openings andcoated with a light-reflective coating thereon.
 1. A light-weightvitreous telescope mirror formed from a one-piece, light-weight,transparent, vitreous telescope mirror blank consisting of a singlevitreous mass and having a pair of oppositely disposed face surfaces anda plurality of separate cavities disposed between said surfaces, one ofsaid face surfaces being provided with a plurality of openings, each ofsaid openings being disposed above and communicating with one of saidcavities, each of said openings being smaller in cross-sectional areathan the cross-sectional area of said cavity, the volume betweenadjacent cavities being occupied by said vitreous mass, said surface ofsaid blank opposite the surface with said openings and coated with alight-reflective coating thereon.